1. Field of the Invention
The present invention relates to building routing tables at intermediate network nodes for routes that cross boundaries of node groups that share routing information, such as routes that cross link-state flooding areas and autonomous systems; and in particular to techniques for passing routes at such boundaries that allow recipient nodes to determine whether a route available across the boundary causes a loop in which the next hop is further from the route destination node than is the recipient node.
2. Description of the Related Art
Networks of general purpose computer systems and specialized devices connected by external communication links are well known and widely used in commerce. The networks often include one or more network devices that facilitate the passage of information between the computer systems and devices. A network node is a network device or computer or specialized device connected by the communication links. An end node is a node that is configured to originate or terminate communications over the network. An intermediate network node facilitates the passage of data between end nodes.
Communications between nodes are typically effected by exchanging discrete packets of data. Information is exchanged within data packets according to one or more of many well known, new or still developing protocols. In this context, a protocol consists of a set of rules defining how the nodes interact with each other based on information sent over the communication links. Each packet typically comprises 1] header information associated with a particular protocol, and 2] payload information that follows the header information and contains information that may be processed independently of that particular protocol. In some protocols, the packet includes 3] trailer information following the payload and indicating the end of the payload information. The header includes information such as the source of the packet, its destination, the length of the payload, and other properties used by the protocol. Often, the data in the payload for the particular protocol includes a header and payload for a different protocol associated with a different layer of detail for information exchange. The header for a particular protocol typically indicates a type for the next protocol contained in its payload. The protocol in the payload is said to be encapsulated in the protocol of the header for the payload.
The headers included in a packet traversing multiple heterogeneous networks, such as the Internet, typically include a physical (layer 1) header, a data-link (layer 2) header, an internetwork (layer 3) header and a transport (layer 4) header, as defined by the Open Systems Interconnection (OSI) Reference Model. The OSI Reference Model is generally described in more detail in Section 1.1 of the reference book entitled Interconnections Second Edition, by Radia Perlman, published September 1999, which is hereby incorporated by reference as though fully set forth herein.
The internetwork header provides information defining the source and destination address within the network. Notably, the path may span multiple physical links. The internetwork header may be formatted according to the Internet Protocol (IP), which specifies IP addresses of both a source and destination node at the end points of the logical path. Thus, the packet may “hop” from node to node along its logical path until it reaches the end node assigned to the destination IP address stored in the packet's internetwork header.
Routers and switches are network devices that determine which communication link or links to employ to support the progress of data packets through the network. A network node that determines which links to employ based on information in the internetwork header (layer 3) is called a router.
Some protocols pass protocol-related information among two or more network nodes in special control packets that are communicated separately and which include a payload of information used by the protocol itself rather than a payload of data to be communicated for another application. These control packets and the processes at network nodes that utilize the control packets are said to be in another dimension, a “control plane,” distinct from the “data plane” dimension that includes the data packets with payloads for other applications at the end nodes.
A link-state protocol is an example of a routing protocol, which only exchanges control plane messages used for routing data packets sent in a different routed protocol (e.g., IP). To reduce the consumption of network resources and improve scalability, some routing protocols divide a large network up into smaller subnetworks. For example, the Open System Interconnection (OSI) protocol suite and the Open Shortest Path First (OSPF) routing protocol divide a network into domains and areas. A domain is a portion of a network under the network administration of a single authority, such as an enterprise or Internet service provider (ISP). A domain is also called an autonomous system (AS). A domain is divided into areas. Each area is a group of contiguous subnetworks and attached end nodes specified by a network administrator, usually manually. In OSI, routers within an AS communicate with each other using an intermediate system to intermediate system (IS-IS) protocol. According to IS-IS, routing within an area (level 1 routing) uses link-state data that distinguishes each link on each router in the area. Routing between areas (level 2 routing) goes through a level 2 router that aggregates the addresses reachable through that level 2 router. By aggregating routing information for addresses reachable over many links of a level 2 router, the amount of network resources consumed to maintain link-state data and make routing decisions can be reduced and network scalability can be enhanced. The division of routers into areas is conventionally a manual process performed by human network administrators.
In an internetwork, networks in different autonomous systems (AS) also route data packets among each other. In general, the network nodes in an autonomous system are manually configured with an Autonomous System identifier (ASID).
Routing information for an AS is summarized at its boundaries with one or more other ASs at intermediate network nodes called border gateway nodes or border gateway (BG) routers. Routing information shared within the borders of one AS is exchanged using an interior gateway protocol (IGP). Example IGPs include the link state protocols OSPF and IS-IS described above. Another IGP, developed by Cisco Systems of San Jose, Calif. for use in its routers, is the Enhanced Interior Gateway Routing Protocol (EIGRP).
A level 3 routing protocol is used to exchange route summary and routing policy information across AS borders. For example, the Border Gateway Protocol (BGP) is a level 3 routing protocol. The BGP sends summary and policy information between adjacent boundary gateway nodes in different ASs using the External BGP (EBGP). The BGP sends summary and policy information between different boundary gateways in the same AS using the Internal BGP (IBGP).
When routes are advertised across boundaries, such as those between areas and autonomous systems, there is a chance for a route originally advertised by one area or autonomous system to be advertised back to that original area or autonomous zone. This can cause loops in which a first router passes traffic across a boundary to a second router farther from the destination than the first router. Both link state and BGP protocols have mechanisms in place to reduce or eliminate loops, so that certain advertised routes that cross such boundaries are ignored if they cause loops. However, these mechanisms may fail in certain cases.
For example, in situations involving many mobile routers that can join and depart a network, routers formerly in the same area or autonomous system may find there is no remaining interior (intra-boundary) route to a particular destination. In some such cases, route information that would ordinarily be rejected by existing mechanisms as causing loops do not in fact cause a loop and could be usefully retained.
For link state protocols there is a requirement that areas not be multiply-connected and that there be no more than two hierarchical levels of connects, such as a hub and spoke arrangement. This combined with a rule for one way passing of routing information prevents loops among areas of an autonomous system.
Based on the foregoing, there is a clear need for techniques to detect and prevent loops in paths that cross routing information boundaries, which do not suffer the problems of prior art approaches. In particular, there is a need for preserving cross boundary, loop-free routes when routers within a boundary are split into separate groups without intra-boundary paths. Also for link-state flooding areas, there is a need for preserving cross area, loop-free routes when areas are not arranged in a hub a spoke.